Many things can cause a blood clot in someone around the age of Secretary of State Hillary Rodham Clinton, 65, doctors said Sunday.

The most likely explanation for her illness is a blood clot in the leg, brought on by her extended bed rest after suffering a concussion earlier this month, said Cam Patterson, a professor and chief of cardiology at the University of North Carolina-Chapel Hill.

However, blood clots called deep vein thrombosis, or DVTs, can form when people are bedridden, which reduces blood flow to the legs, Patterson said.

"When you're not moving around, you don't have as much circulation in your lower extremities, and the blood sits around in your legs longer than usual," Patterson said. "That just makes it more likely that it's going to clot. If blood is moving around rapidly because someone is active, that's less likely."

Although blood clots in the legs are easily treated, they can be dangerous, Patterson said.

Beyond the pain and swelling caused by a blood clot, the real danger is that it can break off from the blood vessel and travel to the bloodstream to the lung, where it can cause sudden death, Patterson said.

If Clinton does have this type of blood clot, however, she is likely to make a full recovery, Patterson said. Doctors typically give patients a blood thinner, such as heparin, as well as several months of anti-clotting drugs such as Coumadin, he said.

With good medical care, a blood clot of this kind "is not likely to recur," Patterson said. "As long as she doesn't end up in another prolonged convalescence. That's good news."

Patterson said he becomes more concerned when a patient develops a blood clot while getting his or her normal amount of exercise. That could signal the patient has an underlying medical condition that needs to be addressed, he said.

The major symptoms of blood clots in the legs are typically swelling, redness and pain, Patterson said. Bedridden people should be alert to these symptoms and seek medical attention if they develop, he said. Blood clots also occasionally form in the legs when people take very long plane trips or have serious illnesses such as cancer.

Patterson said Clinton's illness illustrates how an apparently minor ailment can progress to a life-threatening situation.

Clinton became dehydrated after weathering a gastrointestinal bug, which can cause diarrhea and vomiting. She then became faint and fell, causing a concussion. While the concussion is not likely related to the blood clot, the injury did cause her to spend an extended time in bed. And that bed rest likely led to the blood clot, Patterson said.

"If you think about it, a simple problem like gastroenteritis could have led to sudden death, just through one step after another," Patterson said. Her doctors "need to be saluted," he says. "This is one of the preventable causes of death we as physicians all see every day."

People can reduce their risk of blood clots such as these by getting some exercise every day, even if they are sick in bed, Patterson said. "Even if you don't feel well, get up and walk around a bit, have leg massages and just make sure your legs are moving up and down."